Aluminum wire is an excellent and relatively inexpensive conductor of electricity and the use of aluminum wire is becoming of increasing significance for economic reasons. However, special precautions must be taken in making terminal connections to aluminum wire or overheating is apt to result due to the flow of electric current and a poor connection. Such overheating is frequently sufficient to cause fire and for this reason, the use of aluminum wire for household or industrial wiring has been banned in many communities despite the economy. Although extensive efforts have been made to overcome the problem of overheating, such efforts have so far not met with success.
A number of factors enter into the problem and must be taken into account in making an electrical connection to aluminum wire. One factor is that a film of aluminum oxide (Al.sub.2 O.sub.3) forms practically instantaneously on the surface of aluminum wire when it is exposed to air. This film of aluminum oxide is not only extremely hard, but it also has good insulating properties and makes it difficult to establish a good electrical connection of low resistance to aluminum wire.
Another factor is that the composition of aluminum used in the manufacture of wire varies and such differences will have a bearing on the nature of an electrical connection of a particular wire and the ability to make an electrical connection thereto. In this respect it should be noted that aluminum wire is classified by the advisory authorities as "good" or "bad", with ranges of each type, for the purpose of making electrical connections thereto.
Other factors are that cold flow takes place with aluminum wire and differences in thermal expansion between the wire and terminal may exist which will cause the electrical characteristics of the connections to change during use. Such factors may cause an increase in the resistance of the connection and overheating.
In addition, physical manipulation of the wire during installation or vibration of a wiring device may cause the wire to become loose. Either can result in an increase in the resistance of the connection and overheating.
As noted, any of the above factors may result in detrimental changes in the connection which will cause overheating and load to fire. Generally, when overheating occurs during use, the overheating becomes progressive in nature and thus, terminal connections to aluminum wire present a special hazard.